Allow Home and Dashboards to be uninstalled

Summary:
Ref T7143. This is the simplest fix for adding a new route for Home, at the cost of possibly letting users break instances. However:

• It's kind of hard to get to the option to uninstall Home anyway.
• It's hard to imagine anyone will really uninstall Home by accident, right? Right?
• Put a really scary warning on the action just in case.

Dashboards was only required because Home was required, I think, so just drop that too.

Test Plan: Uninstalled home.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: chad, epriestley

Maniphest Tasks: T7143

Differential Revision: https://secure.phabricator.com/D11753